The present invention relates to a catalyst for automotive lean-burn engine comprising platinum impregnated on a zeolite support and a process for preparing the same, more specifically, to a catalyst for cleaning lean-burn exhaust gas prepared by deposition of platinum metal on a zeolite support by sublimation, which has a widened active temperature window and improved catalytic activity per metallic atom, and a process for preparing the same.
Methods for converting detrimental substances such as CO, HC, NOx and the like contained in the automotive exhaust gas into harmless substances by reduction or oxidation reaction employing a three-way catalyst of platinum-rhodium impregnated on an alumina support have been well known in the art. However, the three-way catalyst has an inherent disadvantage that the air to fuel ratio (A/F) of the exhaust gas has to be set approximately at a stoichiometric ratio to accomplish efficient cleaning. For example, provided that the lean-burn engine air to fuel ratio is leaner than the stoichiometric ratio, the increased content of oxygen in the exhaust gas from the lean-burn engine causes inefficient reduction of NOx to N2, consequently emitting NOx to the air.
Under the circumstance, a NOx storage catalyst which can eliminate NOx at the lean-burn A/F has been proposed as an alternative catalyst. It was, however, proven to be less satisfactory in a sense that the NOx storage catalyst requires extended treating time and highly sophisticated technology to be employed in an engine, which limits its practical application. In this regard, a catalyst which reduces NOx selectively with unburned hydrocarbons has been proposed to realize an automotive lean-burn engine which is highly efficient and consumes decreased amount of fuel as an exhaust gas cleaning system (see: M. Iwamoto and H. Hamada, Catalyst Today, 10:57-71, 1991).
In order to solve the said problems, a copper catalyst prepared by ion-exchange of ZSM-5 zeolite in solution and a platinum catalyst prepared by ion-exchange of ZSM-5 zeolite in solution have been developed in the art, respectively. Although the ion-exchanged copper-impregnated catalyst showed high catalytic activity, it had a disadvantage of low resistance to water and SO2. The ion-exchanged platinum-impregnated catalyst, meanwhile, showed high catalytic activity even at low temperature and high resistance to water as well. However, it still had a problem of a narrow temperature window for maximum catalytic activity.
Under the circumstances, there are strong reasons for exploring and developing a catalyst having a widened temperature window as well as a high catalytic activity for practical application of automotive lean-burn engine.
The present inventors have made an effort to develop a catalyst which can remove polluting substances present in automotive exhaust gas, and found that a platinum-impregnated zeolite catalyst prepared by deposition of active metallic platinum component on a zeolite support through sublimation of platinum under CO flow, has a widened temperature window as well as high catalytic activity even at a high temperature, assuring its application as a catalyst for automotive lean-burn engine.
A primary object of the present invention is, therefore, to provide a process for preparing a catalyst for automotive lean-burn engine.
The other object of the invention is to provide a catalyst for automotive lean-burn engine prepared by the said process.